(a) Field of the Invention
The present invention relates to a thin film transistor (TFT) array panel, and particularly to a transflective TFT.
(b) Description of the Related Art
LCDs are one of the most widely used flat panel displays today. Typically, an LCD includes a liquid crystal (LC) layer interposed between two panels provided with field-generating electrodes. The LCD displays images by applying voltages to the field-generating electrodes to generate an electric field in the LC layer. The electric field determines the orientations of LC molecules in the LC layer, and can be controlled to adjust the polarization of incident light. The polarized light coming out of the LC layer is either blocked or transmitted by a polarizing film. Thus, by controlling the polarization of light, desired images can be displayed.
Depending on the light source that is used, LCDs are classified into a transmissive LCD and a reflective LCD. The light source of the transmissive LCD is a backlight, and the light source of the reflective LCD is ambient light. The reflective-type LCD is usually applied to a small or mid-size display device.
A transflective LCD, which is a combination of the transmissive LCD and a reflective LCD, has been under development and is used in small or mid-size display devices. The transflective LCD is capable of using both a backlight and ambient light as its light source depending on circumstances, and they are usually applied to small or mid-size display devices. The transflective LCD has a transmissive region and a reflective region in a pixel. While light passes through an LC layer only once in the transmissive region, light passes through the LC layer twice in the reflective region. Accordingly, gamma curves of the transmissive region and the reflective region do not match, and images are displayed in different ways between the transmissive region and the reflective region.
To solve the problem, the LC layer may be formed to have different thicknesses (cell gaps) between the transmissive region and the reflective region. However, when the two-cell-gap structure is applied, a thicker layer is formed in the reflective region than in the transmissive region, thereby complicating the manufacturing process. Furthermore, this thickness difference results in the formation of a step between the transmissive region and the reflective region, and the LC molecules are aligned in a disorderly manner around the step. This lack of order in the area around the step degrades image quality. Also, in the area around the step, brightness reversion may occur when the voltage gets high.
As an alternative to the two-cell-gap structure, the transflective LCD may be driven by two different driving voltages depending on whether the LCD is in a transmissive mode or a reflective mode. When two different driving voltages are applied, gamma curves cannot match because there are different critical voltages for transmissive brightness and reflective brightness.
It is desirable to optimize the transflective LCD without the disadvantages mentioned above.